The Effect of Firehose Balance on Current Sheet Formation in Asymmetric Reconnection*

The data from the Magnetospheric Multiscale (MMS) mission has shed new light on the physical processes of reconnection in the magnetosphere. Although much work has been done on reconnection where the background plasma is symmetric across the layer, many of the events MMS observed had significantly asymmetric backgrounds. VPIC simulations show that in the absence of guide fields, even small asymmetries in density can create large changes in the formation of extended current sheets in the diffusion region. We propose that the differential rates of parallel heating on each side of the layer decrease its length by allowing the two sides to reach the firehose condition at different times. Including an initial temperature asymmetry in the simulations to counteract this imbalance restores the elongated current sheet, suggesting that this is indeed the mechanism behind current sheet shortening in asymmetric reconnection.